Method of storing hydrogen in intimate mixtures of hydrides of magnesium and other metals or alloys

ABSTRACT

There is provided an efficient method of storing hydrogen in materials of small mass and small volume. The products of the present invention comprise intimate mixtures (as opposed to alloys) of magnesium and other metals or alloys capable of forming hydrides. The hydrides are selected so that the hydrides have a substantially lower thermal stability than that of magnesium hydride.

RELATED APPLICATIONS

This application is a Continuation in Part of copending application Ser.No. 070175 filed Aug. 27, 1979, now abandoned which in turn is aContinuation in Part of application Ser. No. 770,355 filed Feb. 22,1977, now abandoned.

FIELD OF THE INVENTION

Hydrogen storage of regeneration.

BACKGROUND OF THE INVENTION

Hydrogen is a relatively cheap and clean fuel material which can beutilized in engines, fuel cells, and the like. Unfortunately, thestorage of hydrogen in gaseous form presents many practical problems andsuch storage has, to all intents and purposes, been consideredimpractical for non-stationary uses because of the need for storing thehydrogen in heavy high-pressure cylinders.

Another approach which has received a great deal of attention is the useof reversible metal hydrides. It is well known that many metals formhydrides upon exposure to hydrogen under certain conditions oftemperature and pressure and, upon combined conditions of lower pressureand higher temperature, are induced to dissociate into the metal andhydrogen itself. One of the best known and practical of these metals ismagnesium which forms the hydride MgH₂. The starting material,magnesium, is cheap and provides a very high storage concentration ofhydrogen per unit mass. Unfortunately, the mode of preparation of thehydride requires rather high pressures and temperatures and the uptakeis rather slow. Furthermore, it is stable at ambient temperatures andrather high temperatures are required to dissociate the hydride torelease the combined hydrogen. This stability has two disadvantages.First, outside forces of heat are necessary to generate "start upamounts" of hydrogen before the heating process can be self-sustained;and secondly, there is a substantial wastage involved in keeping thehydride at a temperature high enough to generate the desired quantitiesof hydrogen. Other hydrides have also been investigated. These includethe mixed hydrides of lanthanum and nickel, of iron and titanium, andvanadium. Certain alloys of cerium known as mischmetal pentanickel havealso been investigated. Unfortunately, most of these hydrides have acomparatively low storage capacity and low decomposition temperatures.It would be desirable therefore to provide a hydrogen storage materialwith the stability and storage capacity of magnesium hydride while stillbeing able to provide sufficient hydrogen at relatively low temperaturesfor start up purposes.

SUMMARY OF THE INVENTION

There is provided a class of hydrogen storage materials comprisingmagnesium and other metals or alloys capable of forming hydrides. Thesecond group of hydride forming materials possess one commoncharacteristic and the preferred members, a second one also. They allhave a substantially higher dissociation pressure at given temperaturesthan magnesium hydride; that is to say, they have a lower thermalstability than magnesium hydride and, certain of them, at the same time,will deliver at least one bar of hydrogen pressure at 20° C.

It is the surprising and totally unexpected finding of the presentinvention that, when members of the two components, namely magnesium andother metals or metal alloys within the aforementioned common categoryare mixed together in intimate contact as opposed to in alloyed form,not only are the desired criteria of storage fulfilled, but the totalamount of storage far exceeds that of the individual components at agiven temperature. Thus the compositions of the present invention arehydrogenated under pre-determined conditions of temperature andpressure, permitted to cool, and are storable in a sealed containerwhose pressure resisting capacities need not be substantial since,hydrogen will only be delivered in substantial amounts on theapplication of heat.

In the preferred embodiments, however, at ambient temperature, thecomposition will deliver just enough hydrogen to overcome atmosphericpressure; that is to say, the partial pressure of the mixture at about20° C. will be of the order of one bar, that being sufficient to ignitea burner which in turn will initiate more rapid decomposition of thecomposition to provide enough hydrogen for the desired purposes, that isto say, running a fuel cell, an engine or the like. The great advantageof this arrangement lies in its extreme simplicity and the avoidance ofeither subsidiary initiating heat sources or high pressure storagedevices.

Particularly desirable as metals or alloys of addition can becontemplated such as titanium, iron-titanium, vanadium, lanthanum/nickeland cerium based alloys such as mischmetal pentanickel.

The invention also concerns installations, particularly engines, forusing the hydrogen stored in complex hydrides of the above-mentionedtype. In particular, it includes an arrangement according to which, inorder to supply for example an engine (or other energy receiving ortransforming means) with hydrogen being released from complex hydrideshaving a low thermal stability, i.e., a low equilibriumtemperature--such as those mentioned above with lanthanum-nickel orvanadium--the hydrogen being supplied from a tight hydride storing meansdischarges into a tank supplying the engine, in combination with meanstaking calories from said engine for heating, after start-up, thecomplex hydride to a temperature suitable for the magnesium hydride MgH₂to release its hydrogen.

With such an arrangement engines can be started up from cold, i.e.,without any previous heating.

The invention comprises, apart from the above arrangements, certainothers which are preferably used at the same time and which will beexplained in more detail hereafter.

It relates more particularly to certain applications (particularly forsupplying engines, fuel cells etc.), as well as certain embodiments, ofsaid arrangements; and it relates even more particularly, as novelindustrial products, to the hydrides obtained with the processes of thekind in question and comprising the application of these samearrangements, as well as the installations for carrying on saidprocesses and devices for using the hydrogen stored in the hydrides inquestion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood from the following description ofthe accompanying drawings which are given solely for purposes ofexemplification.

FIG. 1 is a diagram illustrating the absorption of hydrogen at 30 barsof pressure and 345° C. for pure magnesium (dotted line) and intimatemixtures of magnesium with iron titanium, titanium, palladium, andlanthanum pentanickel.

FIG. 2 is a diagram illustrating the absorption of hydrogen by a mixtureof magnesium with lanthanum pentanickel compared with magnesium alone ata pressure of about 60 bars.

FIG. 3 is a similar diagram for a lower pressure of about 30 bars.

FIG. 4 is a similar diagram for the decomposition of the magnesium andlanthanum nickel hydride mixtures under two bars of pressure atdifferent temperatures.

FIG. 5 is a diagram similar to FIG. 2 for a magnesium vanadium mixture.

FIG. 6 is a similar diagram for magnesium/mischmetal pentanickel.

FIG. 7 is a highly schematic indication of the use of the composition ofthe present invention in an engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises the provision of certain novel hydrogenabsorbing compositions. These compositions comprise at least twocomponents. The first component is magnesium. The at least secondcomponent comprises a metal or metal alloy capable of forming hydrides.The hydrides of said at least second component shall have a lesserthermal stability than magnesium hydride. The preferred hydrides have adissociation pressure of at least one bar at 20° C. The utilization ofmore than one metal or metal alloy as the second component is within thepurview of the present invention.

The components are provided initially in finely divided or powderedform. The particle size is not critical, any readily available mesh sizemay be employed, the only provision being that the powder be fine enoughto provide intimate mixing between the two or more components. It hasbeen found desirable to utilize these powders in pellet form; thus thepowders may be compressed together into pellets under pressure. Theamount of pressure is in no way critical, but pressures of the order of5 tonnes/sqcm have been found suitable. If desired, instead of usingpelletized powders, thin sheets of the metals in question may be placedin intimate contact or thin filaments of metals may be woven togetherinto strings, cords or the like. While the form of achieving contact isnot critical and while, during the hydrogenation steps (which will bediscussed in more detail hereinbelow) a certain amount of heat isapplied, the temperature provided to the composition shall not exceedthe melting point of any of the metallic components and, moreover, shallbe below the temperature required to convert any of the previouslyunalloyed components into alloys with magnesium. It has been found that,in the practice of the invention, it is not needful to exceed 350° C.;and even at 450° C., the components will remain in powdered form andwill not become alloyed. In fact, a temperature of 210°-345° ispreferred for hydrogenation.

While the compositions of the present inventions are in no way to beconsidered as limited thereto, the following metals and alloys have beenfound advantageous as constituents of the second component:

Lanthanum nickel (which forms LaNi₅ H₆)

Iron titanium (which forms FeTiH and FeTiH₁.6)

Titanium (which forms TiH₂)

Palladium (which forms PdH₀.3) and

Vanadium (which forms V H₂)

According to another embodiment, magnesium is used in mixture with analloy capable of absorbing hydrogen, one at least of the metal elementsof the mixture being formed by cerium or being in the form of mischmetalpentanickel.

Advantageously magnesium is mixed with a ceric metal based alloyincluding a predominent quantity of cerium.

As ceric based alloy of this kind, the most preferred metal is under thename mischmetal.

It is recalled that mischmetal is defined in the Rompps chemicaldictionary of Dr. Otto Albrecht Neumuller, Frencklische VerlagshandlungStuttgart, 1972, page 2184, as an alloy comprising, in percentages byweight, 50 to 60% cerium, 25 to 30% lanthanum, 15 to 17% neodymium and 4to 6% praseodymium.

Because of its ready availability, the product sold by the firmRhone-Poulenc, S. A., is employed; however, any alloy of the mischmetaltype fitting in with the definition given above which, even if itdiffers a little therefrom, lends nevertheless to hydrides complyingwith the characteristics mentioned above, comes within the scope of theinvention.

The mixture of mischmetal pentanickel with the magnesium allows hydridemixtures to be obtained with high hydrogen capacity per unit of massforming reserves of hydrogen of great purity. By thermal decompositionof the hydrides, the hydrogen may then be readily recovered for thedesired applications.

Considering the low cost of the intermetal compounds of MNi₅ type, theadvantages resulting from the use of an alloy such as defined above canbe readily imagined.

According to a preferred variation for implementing the process of theinvention, the alloy of the mischmetal type employed is used in the formof an intermetal compound with nickel.

Sucb an intermetal compound corresponds to the formula:

    MNi.sub.x

in which M represents the alloy of the mischmetal type and x is a numberfrom 1 to 5, preferably of the order of 5.

The preparation of the intermetal compound of the mischmetal with nickelis carried out in accordance with conventional techniques by melting thecomponents of the alloy, used in the desired stoechiometric proportions,e.g., in an argon atmosphere in an arc furnace. The ratio of magnesiumto second component may lie between 9:1 through 7:3 g corresponding thento alloys containing from 10 to 30% in weight of second component.However, since the magnesium component is far cheaper than the secondcomponent, hence between 10 and 20% by weight of the second component,suitably 20% by weight of the second component, have been found entirelysuitable.

Set forth below in the table is a summary of information concerningintrinsic properties of the components of certain metals which may beutilized for the present invention but should in no way be considered aslimiting. Column 1 shows the melting points of the metals which form thehydrides under discussion. Column 2 shows the temperature at which thevarious hydrides provide a dissociation pressure of 30 bars. Conversely,Column 3 shows the dissociation pressure at ambient temperature or 23°C. It will be noted that certain of these hydrides, namely titaniumhydride, and palladium hydride (in addition to magnesium hydride) show adissociation pressure of less than one bar at 20° C. Clearly suchhydrides would not be suitable for those uses where a starting pressureis essential. However, they may still be employed for their superiortotal storage capacity qualities.

Under desorption conditions, i.e., 350° C./2 bars, all the secondcomponent hydrides may be considered fully dissociated (MgH₂dissociation pressure is of 7 bars at 350° C. and is then the highestlimit for the desorption pressure of the mixtures of the invention at350° C. (with highly purified Mg, the hydrogen pressure is 5.82 bars at345° C. and 6.53 at 350° C.). In order to obtain a satisfactorydesorption rate, lower pressures will be rather used at this temperature(for example 2 bars in curve 4).

The preferred pressure range for hydrogenation of the mixtures isbetween 20 and 60 bars.

    ______________________________________                                                MP of    Temp for   Dissoc.                                           Metal or                                                                              Metal or dissoc. press.                                                                           press. at                                                                            Dissoc.*                                   Alloy   Alloy    of 30 bars 345° C.                                                                       press. at                                  Hydride (°C.)                                                                           (°C.)                                                                             (bars) 20°C. (bars)                        ______________________________________                                        MgH.sub.2                                                                              651     423        5.9    6 × 10.sup.-7                        LaN.sub.5 H.sub.6                                                                     1325     123        967    2                                          FeTiH   1317     89         1460   3.4 (7.9)                                  TiH.sub.2                                                                             1675     315        40     0.3                                        PdH.sub.20.3                                                                          1552     320        48     1.5 × 10.sup.-2                      MNi.sub.5 H.sub.6                                                                     1320     28         1820   24                                         VH.sub.2                                                                              1890     86         8600   1.5                                        ______________________________________                                         *Values for 4:1 M g/metals or alloys are same                                 NA: not available                                                        

EXAMPLES Example 1--Preparation of hydride mixtures from Mg and LaNi₅

A mixture of powdered Mg and LaNi₅ is prepared, in proportions per unitof mass of 80% Mg to 20% LaNi₅. This mixture is then pelletized under apressure of about 5 t/cm².

The pellet (or the pellets) obtained are then degasified in a vacuum,after being previously placed for example in the enclosure intended forthe hydrogen reaction, then the hydrogen is introduced under pressureand at a suitable temperature.

Thus, according to a first embodiment, a pressure P or 60 bars and atemperature T of 345° C. are adopted.

In FIG. 2 are shown the hydrogen H₂ absorption curves, as a function oftime, for Mg alone and for the Mg-LaNi₅ mixture of the invention.

The curves S₁ and S₂ relate to the absorption of hydrogen by simplemagnesium, respectively in pellet and powder form.

Curve C relates to the absorption of hydrogen by the Mg-LaNi₅ mixtureused according to the invention.

The synergistic action of LaNi₅ will be noted which, although in aproportion of only 20%, results in a considerable increase of the speedand the limit of hydridation (curve C).

According to a second embodiment, comparative curves of which are shownin FIG. 3, a lower pressure P, approximately 30 bars, is used and thetemperatures are varied from 210° C. to 345° C.

At this pressure P, it can be seen that powdered magnesium shows afairly low absorption curve S₂, whereas nothing happens with themagnesium in pellet form (curve S₁).

On the other hand, the Mg+LaNi₅ mixture shows, here again, much betterabsorption curves C₁,C₂,C₃,C₄.

After hydridation at 345° C., 30 bars and cooling at 20° C. of a mixtureof 800 g of Mg and 200 g of LaNi₅, Mg is 85% hydrided (51.7 g ofhydrogen are obtained), LaNi₅ is 100% hydrided (2.74 g of hydrogen areobtained), that is to say, 54.44 g of hydrogen are obtained per kg ofmixture.

Under the same conditions, with 800 g of Mg alone, about 25% arehydrided (curve A₂), 15.3 g of hydrogen being obtained, and with 200 gof LaNi₅ alone, at 20° C., (100% hydrided) 2.74 g of hydrogen are fixed.

Into practical energy terms 75 liters of gasoline correspond to 16.7 kgof hydrogen (45 l per 10 kg of hydrogen). Under the conditions ofexample 1, an engine would have to be supplied with 305 kg of Mg/(20%)LaNi₅ mixture (85% hydridation for Mg and 100% for LaNi₅) compared to1,211 kg of LaNi₅ (100% hydridation) or 873 kg of Mg (25% hydridation).

It is to be noted that, during hydridation, the pellets substantiallydouble in volume, but keep relatively good mechanical properties, sothat they can be handled without crumbling.

Finally, complex hydrides or hydride mixtures thus formed (comprisingMgH₂ and LaNi₅ H_(6-x), in which x represents zero or a positive numberless than 6) and suitably conserved in tight containers permit thehydrogen to be supplied at ambient temperatures, at least at thestart-up of the combustion operations to be provided, as will beoutlined below, this property being due to the presence of LaNi₅ hydridewhose equilibrium temperature at which hydrogen, as is known, begins tobe released, is close to the ambient temperature.

Example 2--Preparation of hydride mixtures from Mg and V

A mixture is prepared, in proportions per unit of mass of 80% magnesiumand 20% vanadium in powder form and the mixture is then pelletized as inExample 1.

The pellets thus obtained are placed in the reaction enclosure,degassified in a vacuum and then, as in Example 1 above, hydrogen isintroduced.

FIG. 5 shows, like FIG. 3, the variation as a function of time of thehydrogen absorbed at a pressure P of 30 bars and at differenttemperatures between 265° C. and 345° C.

The results are not modified after numerous absorption-desorptioncycles. They are close to those obtained for the above-mentionedmixture. After 80% hydridation of the magnesium present, the pelletssubstantially double in volume but conserve sufficient cohesion to allowtheir handling.

The aforementioned procedure provides for 80% hydridation of themagnesium and 100% hydridation of the vanadium giving a total hydrogencapacity (measured at 20° C.) of 48.7 g/kg of mixture. 56.2 g ofhydrogen are then obtained with 1 kg of said mixture. Under the sameconditions, 15.3 g of hydrogen are obtained with 800 g of Mg (25%hydridation) alone and 7.5 g of hydrogen are obtained with 200 g of V(100% hydridation) alone.

EXAMPLE 3--Preparation of mischmetal pentanickel powder

A metal block of mischmetal (sold by Rhone-Poulenc, S. A.) was reducedto fine flakes by grating. It was then mixed with nickel in thestoechiometric proportions corresponding to the MNi₅ formula and themixture was pelletized.

The alloy was then melted in an argon atmosphere in an arc furnace. Themelting time was a few seconds. By means of a hydridation cycle, theproduct thus obtained was reduced to powder condition, which facilitatesits handling when mixing with the magnesium in the desired proportions.

A powdery mixture of Mg and MNi₅ was prepared in proportions per unit ofmass of 80% Mg and 20% MNi₅. This mixture was then pelletized at apressure of the order of 5 t/cm².

Example 4--Hydridation of magnesium/mischmetal pentanickel pellets

The pellets obtained in accordance with Example 3 above were placed in areaction enclosure and then degassified in a vacuum.

The hydridation reaction was then initiated by introducing into theenclosure, at a temperature of 345° C., hydrogen at a pressure of 30bars.

A hydride mixture was obtained comprising MgH₂ and MNi₅ H_(6-z), zdesignating a positive number less than 6 or equal to zero.

In FIG. 6, there is shown the hydrogen absorption curves, with respectto time, of the Mg-MNi₅ mixture, and the one obtained with Mg alone. Acomparison of these two curves shows a considerable increase in thespeed and the limit of hydrogen absorption with the hydride mixture ofthe invention.

The procedure provides 87% hydridation of Mg (52.9 g of hydrogen with800 g of Mg) and 100% hydridation of MNi₅ (2.6 g of hydrogen with 200 gof MNi₅) giving a hydrogen capacity (measured at 20° C.) of 55.5 g perkg of mixture (compared to 15.3 g for 800 g of Mg alone) (25%hydridation) plus 2.6 g for 200 g of MNi₅ alone (100% hydridation).

It is to be noted that the experimental values given in thespecification correspond to hydridation periods of 1 hour (see curves 1to 6). The values would be higher with longer periods of time.

In accordance with the above procedure, but using FeTiH or FeTiH₁₋₆, inplace of mischmetal pentanickel, similar results are obtained.

Where, similarly PdH₀.3 or TiH₂ are used, a synergistic enhancement ofhydrogen storage capacity is also provided.

Example 5--Application of the hydride mixture in accordance with Example1 or 4 for supplying hydrogen to hydrogen-consuming engines

The application is carried out in the apparatus schematicallyillustrated in FIG. 7. The hydride mixture or complex hydride formed forexample by MgH₂ and LaNi₅ H₆ or MgH₂ and MNi₅ H_(6-z) is contained in afirst gas tight tank 1 which communicates with an expansion tank 2 whichsupplies the engine 3 with hydrogen.

The pressure in tank 2 is, for example, about 2.5 bars, and the hydrogenflow supplied from the amount of LaNi₅ H₆ contained in the mixture issufficient for starting up the engine.

Heating of the mass stored in tank 1, as shown for example in FIG. 7, isaccomplished with calories drawn from the exhaust system 4 of the engineto provide for the decomposition of MgH₂ which then provides foroperation of the engine. In the operating mode (T being equal to about350° C.) LaNi₅ H₆ or MNi₅ H_(6-z) completely decompose to LaNi₅ or MNi₅,the equilibrium pressure in tank 2 is that corresponding to system MgH₂-Mg+H₂ at the temperature desired (7 bars at 350° C.).

When the engine is stopped, thus allowing tank 1 to cool, the pressureand volume of hydrogen available in tank 2 will be sufficient forreforming LaNi₅ H₆ or MN₅ H_(6-z) which will allow the next start up tobe made.

The hydride mixtures as above disclosed can also be used for supplyingelectrochemical devices, particularly electrochemical cells or fuelcells.

The process described in the above application may be used here again.The heating of tank 1 will be accomplished by Joule effect by using apart of the energy of the cell.

In accordance with the above procedure, but where in place of LaNi₅ H₆or MNi₅ H_(6-z) there is used FeTiH FeTiH₁.6 VH₂, TiH₂ or PdH₀.3 similarresults are obtained, except that in the last two uses external heatsources for startup are advisable.

Whereby, whatever embodiment is adopted, the invention providesimprovement in MgH₂ formation conditions and provides the possibility ofstoring hydrogen in hydride masses, especially those based on MgH₂, withhydrogen pressures and temperatures lower than those necessary up tonow, and with a higher hydrogen power per unit of mass. Moreover, thehydrogen can be supplied at low temperatures owing to the presence ofhydrides of addition, which avoids the necessity of having heatingapparatus for starting.

As it will be understood, and as it results furthermore from what hasbeen said above, the invention is in no wise limited to those of itsmodes of application and embodiments more especially considered; itcovers, on the contrary, all variations.

We claim:
 1. A process of storing hydrogen in a mixture of hydridescontaining magnesium hydride comprising the steps of:a. mixing magnesiumand at least one other metal or alloy or other metals capable ofabsorbing hydrogen under the conditions of steps d and e, said othermetal or alloy being capable of forming hydrides of thermal stabilityless than that of magnesium hydride, and having a dissociation pressureof at least 1 bar at 20° C., the amount of said other metal or alloybeing sufficient to increase the hydrogen absorptive capacity of thetotal mixture over that of the sum of its parts under the conditions ofsteps d and e, the components of said mixture having previously beenpowdered or pulverized; b. pelletizing thus produced powder; c. vacuumdegasifying the thus produced pellets; d. placing the pellets in anenclosed empty vessel; and e. providing hydrogen to said vessel at apredetermined temperature below 350° C. and the melting point of saidpelletized mixture and pressure of 20-60 bars to permit absorption ofthe hydrogen upon the pellets.
 2. The process in accordance with claim 1wherein the said other metals or alloys selected from the groupconsisting of lanthanum/nickel, vanadium, mischmetal/nickel, oriron/titanium.
 3. A process in accordance with claim 1 where thehydrogen is provided at a temperature of about 210° to about 350° C.